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Eur. J. Mineral., 32, 67–75, 2020 https://doi.org/10.5194/ejm-32-67-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Raman spectroscopic identification of cookeite in the crystal-rich inclusions in spodumene from the Jiajika lithium pegmatite deposit, China, and its geological implications Xin Ding1, Jiankang Li2, I-Ming Chou3, Zhenyu Chen2, and Shenghu Li4 1State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Beijing), Beijing 100083, China 2MNR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China 3CAS Key Laboratory of Experimental Study Under Deep-sea Extreme Conditions, Institute of Deep-sea Science and Engineering, Chinese Academy of Sciences, Sanya 572000, China 4Shandong Institute of Geological Sciences, Ji’nan 250013, China Correspondence: Jiankang Li ([email protected]) Received: 1 January 2019 – Accepted: 18 September 2019 – Published: 16 January 2020 Abstract. Cookeite usually occurs as a late alteration product in lithium–cesium–tantalum-type granitic peg- matite. Consequently, cookeite-bearing crystal-rich inclusions (CIs) in pegmatite are considered to be of sec- ondary origin, which constrains our understanding of pegmatite formation. Thus far, no reported cookeite has produced a distinct Raman spectrum. However, the CIs hosted in spodumene in the Jiajika pegmatite de- posit, China, contain a cookeite-like hydrous lithium–aluminum–silicate phase, yielding a distinct Raman spec- trum. In electron microprobe analysis, focused ion beam scanning electron microscopy, and time-of-flight sec- ondary ion mass spectrometry (ToF-SIMS), the average composition of this hydrous phase was determined as Li1:005.Al3:997Fe0:018/.Si3:086Al0:914/O10:076OH7:902F0:098, close to the International Mineralogical Association (IMA) formula of cookeite, (Al, Li)3Al2(Si, Al)4O10(OH)8. The distinct Raman peaks at 98, 167, 219, 266, 342, 382, 457, 592, 710, and 3640 cm−1 were consistent with those of natural cookeite recrystallized in a hydrother- mal diamond-anvil cell. The peaks were ascribed to the crystallization of cookeite from the liquid trapped in the closed space during the spodumene crystallization, which occurred at relatively high temperature and pressure without incorporating the minor elements commonly present during alteration processes. These minor elements often obscure the Raman signals, primarily by fluorescence effects. This type of cookeite in CIs with distinct Raman signals is unusual and can indicate whether the cookeite crystallized from fluid trapped within the closed space of a primary inclusion. In such a case, the fluid can be considered a flux-rich hydrous melt in pegmatite formation models. 1 Introduction 1970; Heinrich, 1975; London and Burt, 1982; Bobos et al., 2007; Novák et al., 2015). Cookeite is occasionally found Cookeite is an uncommon member of the chlorite group, in hydrothermal veins or hydrothermally altered sedimen- tary rocks (Vidal and Goffé, 1991). It also occurs in crystal- with the IMA formula LiAl4(Si3Al)O10(OH)8 (Anthony et al., 1995). Most cookeite is a late hydrothermal alteration rich inclusions (CIs), which characterize the fluid inclusions product of spodumene, petalite, and other Li-rich minerals, in granitic pegmatites (Roedder, 1992). Because cookeite is which form in granite pegmatites at low temperatures (Cerný,ˇ conventionally thought to have formed in the late-stage al- Published by Copernicus Publications on behalf of the European mineralogical societies DMG, SEM, SIMP & SFMC. 68 X. Ding et al.: Raman spectrum of cookeite teration processes, its presence in CIs implies a secondary origin of the CIs (Anderson et al., 2001; Anderson and Mac- carron, 2011; Anderson, 2013). Despite its common occur- rence, no natural cookeite, including CI-enclosed cookeites in pegmatite environments, is known to produce distinct Ra- man signals. Consequently, no characteristic Raman spec- trum of cookeite has been reported. In the database of Ra- man spectra, X-ray diffraction, and chemical data (RRUFF), the Raman spectra of cookeite are very noisy, with no distinct peaks (Downs, 2006). Therefore, cookeite has been identified mainly by analyzing its composition or by semiquantitative X-ray spectroscopy (London, 1986; Anderson and Maccar- ron, 2011). However, the Jiajika pegmatite-type lithium deposit in western Sichuan, China, contains a cookeite-like hydrous solid phase within spodumene-hosted CIs, which yields a distinct Raman spectrum (Fig. 5 in Li and Chou, 2016). In the present study, this hydrous solid phase is confirmed as be- ing cookeite by further analyses, including with an electron Figure 1. Photomicrographs of spodumene-hosted crystal-rich in- microprobe (EMP), focused ion beam scanning electron mi- clusions (CIs) in the Jiajika deposit, showing one fluid inclusion croscope (FIB-SEM), and time-of-flight secondary ion mass assembly (FIA) of CIs with a uniform composition and the crys- spectrometer (ToF-SIMS); its Raman signals are compared tal/fluid proportion. Crt – cristobalite; Spd – spodumene; Zab – with those of a natural cookeite recrystallized in a hydrother- zabuyelite; Cal – calcite, Qz – quartz; Ck – cookeite. mal diamond-anvil cell (HDAC). Given the unusual Raman feature of the cookeite in the CIs from the Jiajika pegmatite, Mn)PO ). The spodumene crystals are white or off-white we infer an origin different from those of secondary CIs (An- 4 euhedral plates 5–10 cm in length and 1–5 cm in width and derson et al., 2001; Anderson and Maccarron, 2011; Ander- are clearly in contact with quartz and albite crystals (Li and son, 2013). The cookeite might have formed from fluid origi- Chou, 2016). The late alteration is relatively weak in the nally trapped during the crystallization of spodumene. There- spodumene pegmatite, and occasionally spodumene was re- fore, this fluid can be considered to be a primary flux-rich placed by albite in myrmekitic texture at the contact face with hydrous melt in pegmatite formation models (London, 1999, microcline. Currently, cookeite crystals, formed through late 2008, 2018; Thomas et al., 2000, 2009, 2011a, b). In this hydrothermal alteration of spodumene at low temperature, paper, we suggest that cookeite in CIs with distinct Raman were not observed in the pegmatite dikes. signals is a viable indicator of the primary nature of CIs in In the spodumene pegmatite in the Jiajika deposit, the CIs pegmatite. of the spodumene often contain a hydrous solid phase that has been identified and imaged with Raman spectroscopy (Figs. 1, 2; Li and Chou, 2016). Electron microprobe (EMP) 2 Features of cookeite-like phases within the analyses suggest a cookeite composition of this phase (Li and crystal-rich inclusions in the Jiajika pegmatite Chou, 2016). The CIs, which often occur as isolated indi- deposit viduals or in-fluid inclusion assemblages (FIAs) with similar composition and crystal/fluid proportion, are considered to The Jiajika granitic pegmatite in western Sichuan, China, is be primary in origin (Fig. 1). The exceptions are FIAs of CIs the largest lithium deposit in Asia (Li et al., 2013a). In this that are cross-cut by late stage CO –H O–NaCl and aqueous deposit, the pegmatite dikes radiate horizontally and verti- 2 2 fluid inclusions (Li and Chou, 2016, 2017). The primary CIs cally around the two-mica granite intrusion. With increas- are 20–100 µm long and 10–20 µm wide and have a subhedral ing distance from the granite, the mineralogy of pegmatite to euhedral negative spodumene crystal shape. Within the dikes change from microcline pegmatite, to microcline-albite CIs, the cookeite-like phase is commonly accompanied by pegmatite, albite pegmatite, spodumene pegmatite, and lep- semi-euhedral crystals of zabuyelite, cristobalite, and quartz idolite (muscovite) pegmatite. The two-mica granite and (Fig. 2; Li and Chou, 2016). It coexists with a CO phase, pegmatites are hosted in schists formed by metamorphic 2 and occasionally coexists with an aqueous phase (Figs. 1 overprint of early Triassic mudstones and sandstones (Li and 2). In CI heating experiments, the cookeite-like phase, et al., 2007). The spodumene pegmatite dikes are the main zabuyelite, cristobalite, and quartz dissolve and melt at 400– lithium ore bodies; they are mainly composed of spodumene, 600 ◦C, and the CIs are homogenized into a carbonate-rich quartz, albite, muscovite, and a few rare metal minerals aqueous fluid at 500–700 ◦C (Li and Chou, 2017). of columbite, beryl, tantalite, thorite, and sicklerite ((Li, Eur. J. Mineral., 32, 67–75, 2020 www.eur-j-mineral.net/32/67/2020/ X. Ding et al.: Raman spectrum of cookeite 69 mounted on a TESCAN LYRA instrument platform (TOFW- ERK AG, CNNC Beijing Research Institute of Uranium Ge- ology). In this analysis, the ion beam energy and current were set to 15 keV and 200 pA, respectively. The staying time and milling depth on the crystal surface were 10 µs and ∼ 0:2 µm, respectively, over an area of approximately (10 × 10) µm2. To prove that the hydrous solid phase in CIs is indeed cookeite, we recrystallized a natural cookeite sample from Minas Gerais pegmatite, Brazil (Catalogue No. 115846 00, National Museum of Natural History, Smithsonian Institu- tion, USA), in a hydrothermal diamond-anvil cell (HDAC, type HDAC-VT; Li et al., 2016). The cookeite sample con- tained 46.63 wt % Al2O3, 35.77 wt % SiO2, 0.62 wt % F, 0.24 wt % MnO, 0.09 wt % SnO, 0.08 wt % SrO, 0.03 wt % Cs2O, 0.03 wt % Pb, 0.02 wt % K2O, 0.01 wt % Cl, which were analyzed with EMP at the conditions described above. Following Li et al. (2013b), the preheated cookeite sample and pure water were sealed together with an air bubble in the HDAC sample chamber. The sample chamber is a hole Figure 2. Crystal-rich inclusions (CIs) hosted in spodumene from (of diameter 0.5 mm) at the center of a Re gasket (of diam- the Jiajika deposit.